The invention relates to apparatuses and methods for detecting partial discharge on a power cable. The apparatuses and the methods are particularly useful for detecting partial discharge on installed high-voltage power cables. More particularly, the apparatuses relate to a partial discharge detection test link and a partial discharge detection system.
For satisfactory long-term performance extruded dielectric power cable systems at stress levels used in modern extra-high-voltage cross-linked polyethylene (XLPE) cable systems, it is necessary to maintain high purity levels of material and precision of manufacture. If not, one or more latent defects may occur within the dielectric. For quality assurance, the cables themselves as well as the accessory components are rigorously tested electrically prior to installation. However, defects can subsequently be introduced. Such defects include external damage to cables or contamination thereof as well as mishandling or misalignment of accessory components, particularly during installation.
Latent defects of this type can be detected by conducting a post installation electrical test. One such test, considered most sensitive and revealing, measures partial discharge (PD) of the field-installed accessories. This test energizes the power cable system with alternating voltage, at or near power frequency, and at a working voltage or a small overvoltage.
It is impractical to conduct this test for long length of high voltage power cable using a portable test voltage source. As a result, tests on installed power cable circuits are often conducted by connecting the circuit to be tested to the electrical power cable system and monitoring its performance for a period of time. Upon satisfactory completion of the monitoring period, the circuit is then placed into commercial service.
Power cable systems are sometimes installed without built-in PD measurement sensors in the accessories. If a PD measurement test is to be conducted on such a circuit, it is necessary to provide PD measurement sensors for sensing PD signals. Usually, the PD measurement sensors are releasably connected to the service links. With reference to FIG. 1, a partial discharge sensor 2 is releasably disposed about a service link 4. The service link 4 is attached to an accessory 6 in a form of a termination. The accessory 6 includes a first accessory component 8, a second accessory component 10 and a sleeve 12 which connects the first accessory component 8 and the second accessory component 10 to each other. Each of the first and second accessory components 8 and 10 has a lug 14. Opposing ends of the service link 4 are respectively connected to the lugs 14 by fasteners 15 such as screws or bolts. By way of example only, the first accessory component 8 is connected to switchgear 16 while a high-voltage cable 18 is connected to the second accessory component 10.
The service link 4 is required to be added to the accessory when the power cable system is in service in order to control voltages occurring on the power cable sheath by allowing flow of induced power frequency and transient currents. The service link 4 can be fitted in a variety of configurations to control the magnitude of the flowing current. As illustrated in FIG. 1, the service link 4 is fitted directly to the accessory 6. Also, service links can be used at remote enclosures, such as link boxes or link kiosks, which are normally located with in a few meters of the accessories.
Many hazards are associated with energized power cable systems. One hazard is standing power frequency voltages that exist between service links and from service links to earth which arises from normal operation of the power cable system. Another hazard is transient voltages that exist between service links and from service links to earth which arises from switching operations, lightning strikes and the like. Yet another hazard is severe electrical arcing as a result of flashover involving the service links. Thus, there are three major considerations of safety regarding testing of energized power cable systems. One, conducting partial discharge tests on an energized power cable system should pose no risk of injury to living beings. Two, there should be no added risks of damage to the circuit being tested or equipment associated with the tests. Three, faults are likely to occur within the first few hours of the operation of the power cable system when PD testing is likely to be in progress.